Group III-Nitrides have gained a lot of importance for the last decade, for example in semiconductor processing. Examples of applications are High Electron Mobility Transistors (HEMT) for high power and high frequency applications, blue LEDs, etc.
Because monocrystalline group III-Nitride substrates are not commercially available so far, a lot of effort has been put in optimizing growth of such group III-nitrides on appropriate substrates. The most commonly used substrates for growing group III-nitrides on are silicon, sapphire and silicon carbide. However, the growth or deposition of group III-nitride on these substrates suffers from large lattice and thermal mismatches, making the growth of high crystal quality material difficult. Table 1 shows material properties of commonly used substrates for group III-Nitride growth, as well as theoretical thermal and lattice mismatches with respect to GaN. Also shown in this table are the properties of Ge and the corresponding theoretical lattice and thermal mismatch with GaN.
TABLE 1Material properties of GaN, Sapphire, 6-HSiC, Si(111), Ge(111), AlN,GaN, InN and a reference Ge(111). The lattice and thermal mismatches aregiven with respect to GaN.LatticeThermalThermalLatticemismatchExpansionmismatchThermalconstant ato GaNcoefficientto GaNConductivityBandgapMaterial(Å)(%)(10-6 K-1)(%)(W/cmK)(eV)GaN3.190.05.601.33.39(reference)Sapphire2.7516.07.5−340.338.06-HSiC3.083.54.2254.92.36Si(111)3.84−16.92.6541.31.12Ge(111)4.00−20.35.9−5.50.580.66AlN3.11−22.34.2−28.82.06.0GaN3.19−20.35.6−5.51.33.39InN3.53−11.83.8−35.60.540.6-0.8Ge(111)4.005.90.580.66(reference)
Because of the high thermal mismatch between GaN and Silicon Carbide (SiC) or Silicon (Si) substrates, the growth of GaN on these substrates results in high tensile thermal stress which may lead to the formation of cracks in the GaN layer after cooling down. Especially in the case of Si substrates, a buffer layer between GaN and Si has to be implemented to accommodate for the tensile thermal stress.
Before the growth of GaN on the above mentioned Si or SiC substrates, an extra or intermediate layer is required. For example, GaN cannot be grown directly on Si, because of the occurrence of an etching reaction between the Si of the substrate and the Ga atoms of the GaN layer deposited on the Si substrate. Therefore an extra layer, for example an AlN layer, is grown in between GaN and Si. According to another example, because wetting properties of GaN on SiC are not good, first AlN is grown on the SiC substrate before GaN is deposited on that substrate. The presence of AlN on SiC also reduces the lattice mismatch between GaN which is later on deposited and the SiC substrate and improves the wetting properties. Hence, AlN grows better on SiC than GaN, because of better wetting properties. Without an intermediate layer, GaN grows 3-dimensionally on SiC.
In case of Sapphire substrates, the introduced stress is also large but compressive. Nevertheless, compressive stress can also lead to the formation of cracks in the substrate. Direct growth of GaN with good crystal quality on sapphire is not possible.
Furthermore, Sapphire and Silicon Carbide are difficult to heat up uniformly, because of the high bandgap of respectively 8 eV and 2.36 eV. The bandgap of Silicon, which is 1.12 eV, allows uniform heating, but a high temperature is needed to remove the native oxide layer.
In EP 1 548 807 deposition of group III-nitride material onto a silicon substrate with a porous top layer has been described. An intermediate layer comprising Ge, preferably with a Ge concentration which is increasing in a direction away from the substrate, is first provided onto the substrate before a layer of a group III-nitride material is provided onto the substrate. The Ge-comprising intermediate layer may preferably be SiGe. The Ge-comprising intermediate layer is used to reduce the thermal stress and is a protective layer against oxidation and nitridation of the porous top Si layer. The porous top Si layer reduces the large lattice mismatch between the Si and the group III-nitride.
The method described in EP 1 548 807 thus requires deposition of an extra layer onto the substrate before a group III-nitride can be deposited onto the substrate.
However, the use of buffer or intermediate layers forms a barrier between the material of the substrate and the nitride layer. This may be a disadvantage when the group III-nitride/substrate structure is intended to be used in e.g. semiconductor devices.
From table 1 it can be seen that the thermal mismatch between GaN and Ge is −5.5%, which is small compared to the other substrates mentioned in the table. For group III-nitrides such as e.g. GaN, InN and AlN the thermal expansion coefficients are respectively 5.6, 3.8 and 4.2. GaAs has a thermal expansion coefficient of 5.7 and Ge has a thermal expansion coefficient of 5.9. Hence, for minimising the thermal mismatch, GaN may be the best choice to grow on Ge. Also GaAs would be a good substrate with Ge on top to grow GaN on. Hence, using Ge as a substrate to deposit GaN on, or in general to deposit a group III-nitride on, would result in limited additional thermal stress during cooling down after growth. This is especially the case for nitrides that contain a high Ga concentration.
The growth of group III-nitrides, and especially of GaN, on a Ge substrate would be interesting for different reasons. For example, as already mentioned above there is a small thermal mismatch between group III-nitride, such as GaN or InGaN and AlGaN with high Ga content, and Ge. Furthermore, Ge has a relatively low price. Furthermore, Ge has a low bandgap (0.66 eV) and can be heated up uniformly and reproducibly in a deposition system using radiative heating, for example molecular beam epitaxy (MBE). The possibility for uniform heating of the Ge substrate may be advantageous during deposition of the group III-nitride layer onto the substrate to form uniform layers of group III-nitride material, e.g. GaN, with good crystal quality. However, a problem that arises is that the lattice mismatch between Ge and group III-nitrides, e.g. GaN, is larger than in case of commonly used substrates such as Silicon, Sapphire or Silicon Carbide. A large lattice mismatch can lead to growth of group III-nitrides, e.g. GaN, with bad crystal quality, which cannot be used in semiconductor devices.
In “Journal of Crystal Growth, 279, (2005), p. 311” E. Trybus et al. describe the growth of InN on a Ga-doped Germanium (111) substrate via plasma assisted molecular beam epitaxy. A lattice mismatch of 11.3% was observed between the InN layer and the Ga-doped Ge(111) substrate. The crystallographic structures of the InN layer formed were studied with double-crystal X-ray diffraction (XRD). The best obtained full width at half maximum (FWHM) values for InN were ˜144 arcseconds and the best rocking curve measurements showed ˜2597 arcseconds FWHM, indicating significant tilt and mosaic grain structure. Furthermore, it was shown by diffraction contrast microscopy measurements that a 0.4 μm thick InN film contains a high density of threading dislocations and grain boundaries.
In the InN/Ge structure formed in the above-described document, a high lattice mismatch exists between the InN layer and the Ge substrate. Such a high lattice mismatch may lead to many defects. For example a thermal mismatch between InN and Ge of −36% can lead to additional stress of the InN layer after deposition, e.g. during cooling down. Furthermore, the InN/Ge structures formed show a rough interface between the InN layer and the Ge substrate. Probably this is due to mixing of the Ge and the InN because of an eutectic reaction between In and Ge. The FWHM of the XRD omega-2theta scan is ˜144 arcseconds, and the rocking curve FWHM˜2597 arcseconds. The latter shows the relatively poor crystal quality of InN directly grown on Ge(111) when compared to values of less than 300 arcseconds for InN grown on sapphire. Direct growth of good quality InN on Ge may thus be difficult because of the eutectic reaction between In and Ge. Because of this, these structures are less suitable to be used in e.g. semiconductor devices.
Up till now, no satisfactory methods have been developed for depositing group-III nitrides and especially GaN onto a Ge substrate in a cost-effective way and such that the group III-nitride/Ge structures formed are suitable for use in e.g. semiconductor devices.